1. Field of the Invention
The present invention generally relates to a rotating storage system, and more particularly to a disk drive system having an adaptive resonance mode cancellation servo.
2. Description of the Related Art
A disk drive sector servo system with a 500 Hz open loop crossover frequency can meet a 15 kTPI (tracks per inch) track-following accuracy requirement.
However, the growth of track density to higher than 25 kTPI (e.g., expected by year 2000) has emerged as a major challenge to the actuator and servo system design. Mechanical system resonance is one of the limiting factors to obtaining higher bandwidth control. Using micro-electromechanical devices has been studied to increase actuator response characteristics.
Innovation in the actuator system design to increase the servo-crossover frequency is desirable. However, the storage industry needs cost-effective innovations in servo system design methodology that preserve the time-proven actuator system concepts while meeting the high track density requirements.
There are several actuator resonance modes found in a 3.5xe2x80x3 rotating storage system. The first important mode occurs around 3 kHz, and is understood to arise from bending of the actuator voice coil motor (VCM) around its pivoting point. This bending mode characteristic is sensitive to temperature, pivot parameters and other properties of a disk drive.
A conventional approach to managing the presence of this first mode has been to introduce a xe2x80x9cdeepxe2x80x9d digital notch filter in series with the servo controller during a seek and track-follow mode. A notch filter (or sometimes simply referred to as a xe2x80x9cnotchxe2x80x9d) reduces the negative effect of a rise in the transfer function gain that occurs due to the coil bending resonance (CBR).
Due to the temperature-induced drift of the resonance frequency, as well as the manufacturing variability encountered within a manufactured product population, the conventional digital notch filters have been designed to have broader-than-required attenuation bandwidth (e.g., an xe2x80x9cover-designxe2x80x9d or an xe2x80x9coverkillxe2x80x9d of the mode), thereby resulting in a corresponding phase loss in the crossover region of the servo loop. The loss of phase in turn limits the achievable crossover frequency of the track-follow servo system.
Thus, using a traditional xe2x80x9cdeepxe2x80x9d notch to handle the operating requirement has become a limiting factor.
It is noted that the conservative xe2x80x9cdeepxe2x80x9d notch design can be somewhat favorably enhanced by conducting tests in a manufacturing line for each product. However, the design still produces excessive phase lag because of having to compensate for temperature-induced drift. Further, it cannot effectively support high track density products beyond 25 kTPI. This is unacceptable given that the track density (TPI) is expected to be well above 25 kTPI in 2000 and beyond.
Further, it is noted that adaptive control methods including system identification techniques have been theoretically proposed to optimize servo systems. However, because of the mathematical complexity and signal processing power required to implement these techniques, these theoretical methods have not been implemented or designed yet for the conventional servo system architectures found in present storage products.
Further, the conventional adaptive control solutions attempt to solve a broader problem of model structure, model order, and controller parameter determination simultaneously, thereby leading to a computationally demanding and intensive approach.
Additionally, no low cost signal generation capability exists in the conventional systems and methods.
In view of the foregoing and other problems of the conventional methods and structures, an object of the present invention is to provide a method and structure in which improved servo bandwidth and decreased phase loss are achieved.
Another object is to develop a simpler and implementable solution, as compared to the conventional systems, such that the known aspects of the system are incorporated innovatively so that the unknown parameter set that must be determined within a product setting becomes manageable.
For example, if a resonance mode is known to exist, then it is sufficient to search for the mode within a pre-specified frequency range instead of treating it as a completely unknown parameter.
Thus, yet another object of the present invention is to provide a simple and effective method to solve the resonance uncertainty problem as described above.
In a first aspect of the present invention, a methodology is provided to optimize control system properties by detecting and configuring a resonance mode cancellation filter.
In the first aspect, the methodology (and apparatus) includes designing a set of digitally selectable optimum resonance cancellation filters, generating a series of excitation signals for input to the filters, generating a characteristic resonance frequency based on the excitation signals, and computing an address pointer corresponding to the resonance frequency, to select an optimum resonance cancellation filter.
In a first embodiment of the present invention, an optimum resonance mode cancellation solution is proposed using a xe2x80x9cshallowxe2x80x9d notch which is more effective and efficient in contrast to a conservative xe2x80x9cdeepxe2x80x9d notch. That is, the xe2x80x9cshallowxe2x80x9d notch provides a phase margin improvement of about 20% at around the 500 Hz crossover region.
The method of the first embodiment of the present invention shows that the conservative xe2x80x9cdeepxe2x80x9d notch design that produces about a 10-degree phase lag can be replaced by an xe2x80x9coptimumxe2x80x9d notch with a phase lag of less than 3 degrees with a software-based solution, without any actuator modification. The phase enhancement improves the error rejection capability of a sector servo system which allows higher track density storage products to be realized. The increased phase margin can alternatively be traded-off to achieve increased open-loop crossover frequency.
Additionally, the present inventors have found that an optimum mode cancellation solution requires close matching of the CBR, which leads to a second embodiment of the present invention that relates to resonance frequency determination.
In the second embodiment, the invention provides a comprehensive CBR frequency determination and mode cancellation method which requires an in-situ method to generate a near-sinusoidal signal at low cost for resonance excitation, a least square algorithm to determine the resonance frequency location and an address pointer to extract the stored notch parameters, all using available product resources. These functions are achieved without lengthy or complex mathematical operations. Methods have been developed, tested and demonstrated on two separate storage product platforms to validate the method and structure of the present invention.
In a third embodiment, alternative ways to further simplify the implementation requirements of the present invention are provided by utilizing a priori known CBR characteristics.
Specifically, the present invention identifies the predictable dependency of CBR frequency on temperature and eliminates a xe2x80x9cwiderxe2x80x9d and xe2x80x9cdeeperxe2x80x9d conservative fixed frequency notch by adaptively selecting pre-designed and pre-stored optimum notch parameters (e.g., in a table or the like) based on an internal temperature sensor measurement.
The temperature sensor-based resonance mode prediction is fine-tuned further by an in-situ learning process in which the temperature vs. resonance sensitivity relationship over a period of time during the service life of a disk drive is captured and stored such that long-term system variations are gradually compensated.
An optimal location of the temperature sensor to provide accurate resonance data is also defined. Further, the invention may be embodied as a computer implementable program stored in a signal-bearing medium.
With the unique and unobvious aspects of the present invention, wider servo bandwidth and decreased phase loss result, with a simple system not requiring a xe2x80x9cdeepxe2x80x9d notch. Along these lines, no xe2x80x9coverdesignxe2x80x9d is required to process a mode. That is, the notch parameters are optimized to the mode to provide a substantially linear function of notch characteristics to mode frequency.
Thus, the present invention easily and efficiently optimizes control system properties by detecting and configuring a resonance mode cancellation filter with an optimum notch for each mode frequency.